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Public Health Fact Sheet

Influenza: Pigs, People & Public Health

Authors: Amy L. Vincent, DVM, PhD, USDA-ARS National Animal Disease Center; Marie R. Culhane, DVM, PhD, 

College of Veterinary Medicine, University of Minnesota; Christopher W. Olsen, DVM PhD, School of Veterinary 

Medicine and School of Medicine and Public Health, University of Wisconsin-Madison. 

Reviewers: Jeff B. Bender, DVM MS, University of Minnesota; Andrew Bowman, DVM PhD, The Ohio State University; 

Todd Davis, PhD, CDC; Ellen Kasari, DVM MS, USDA; Heather Fowler, VMD PhD MPH, National Pork Board

Background:

Influenza viruses exist in four “types,” designated A, B, C, and D. 

Of these, only influenza A viruses are significant concerns for the 

health of pigs, whereas influenza A and B are of concern to people. 

There are a large number of different “subtypes” of influenza A 

viruses. These subtypes are classified by the hemagglutinin (H or 

HA) and neuraminidase (N or NA) proteins of the virus. These are 

also the proteins against which the host directs antibodies that can 

neutralize the virus, and are, therefore, the major target of vaccines. 

Of practical significance, antibodies against one IAV subtype 

provide limited cross-protection to IAV of different subtypes.

There are at least 18 different subtypes of hemagglutinin and 

11 different subtypes of neuraminidase among influenza A 

viruses. Subtypes are distinguished by differences in their genetic 

sequences, which translate into differences in protein structure 

and antibody recognition sites, and potentially their infectiveness 

and pathogenicity. The combination of HA and NA subtypes are 

depicted by H and N designations, such as H1N1, H3N2, etc. In 

the course of history, relatively few HA and NA combinations 

have consistently circulated among pigs or people (predominantly 

H1N1, H1N2 and H3N2 in pigs, and H1N1, H2N2 and H3N2 in 

people). In contrast, virtually all of the possible influenza A virus 

subtypes exist among waterfowl, with the exception of subtypes 

H17, H18, N10, and N11 that have only been detected in bats. In 

birds, influenza viruses predominantly infect the gastrointestinal 

tract rather than the respiratory tract, which is the target organ 

in pigs, people, horses and other mammalian hosts of influenza 

viruses. The infections in waterfowl generally do not make the 

birds sick. In waterfowl, the viruses are shed in the feces and, 

ultimately, into lakes, ponds and other environments that are 

visited during migrations. These environments may also include 

farm grounds where livestock or poultry are raised.

IAVs carry 8 separate gene segments of ribonucleic acid (RNA), 

rather than on one single genetic molecule. In addition to 

undergoing genetic change through mutations in their RNA genes, 

#04726 09/18 ©2004, 2018 National Pork Board, Des Moines, IA USA. This message funded by America’s Pork Producers and the Pork Checkoff.

Influenza A viruses (IAV) were first isolated from swine in the United States in 1930. Since that time, they remain an economically 

important cause of respiratory disease in pigs throughout the world and a public health risk. The clinical signs/symptoms of influenza in 

pigs and people are remarkably similar, with influenza-like illness (ILI) consisting of fever, lethargy, lack of appetite and coughing prominent in both species. Influenza viruses can be directly transmitted from pigs to people as “zoonotic” disease agents (pathogens that are 

transmitted from animals to humans or shared by animals and humans) and cause human infections. Conversely, influenza viruses from 

people can also infect and cause disease in pigs. These interspecies infections are most likely to occur when people and pigs are in close 

proximity with one another, such as during livestock exhibits at fairs, live animal markets, swine production barns, and slaughterhouses. 

Finally, pigs can serve as intermediaries in the generation of novel reassorted influenza viruses since they are also susceptible to infection 

with avian influenza viruses. These novel viruses may contain new combinations of gene segments from avian, human and mammalian 

influenza viruses with the potential to be transmitted to people. Importantly, reassortant viruses were the cause of the human influenza 

pandemics in 1957, 1968, and 2009, with different combinations of human, avian, or swine viruses in each event. Finally, replication of 

avian lineage IAVs in pigs may also allow them to adapt and efficiently infect mammals, ultimately leading to a virus with greater potential to transmit to, infect and cause disease in people even without reassortment with mammalian viruses. 

Veterinarians can help pig producers design farms, develop management protocols, and devise personnel policies to minimize movement 

of IAV between pig populations. Protocols regarding the frequency and source of pig introductions into farms and the isolation and/or 

quarantine of newly introduced animals may have the potential to reduce the number of diverse IAVs in a pig population. Additionally, 

personnel policies such as those recommended below may assist in the prevention of interspecies transmission of influenza viruses. 

Therefore, veterinarians are uniquely positioned to provide advice and intervention strategies to minimize influenza disease risk that can 

afflict both swine and humans.
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the segmented genome of the virus has important implications for 

virus evolution. If two (or more) influenza viruses simultaneously 

infect cells in a single host, then these viruses can exchange RNA 

segments with one another during replication, thereby creating 

viruses with entirely new combinations of genes. This process, 

known as reassortment, was the basis for the emergence of viruses 

that caused influenza pandemics in the human population in 1957, 

1968, and 2009. In 1957 and 1968, influenza viruses from waterfowl 

reassorted with the previously circulating human influenza viruses 

to create viruses with different hemagglutinin subtypes (from H1 

to H2 in 1957 and from H2 to H3 in 1968). However, in 2009, 

the reassortment that generated the human pandemic virus was 

between viruses from a North American swine lineage termed 

“triple reassortant” and virus genes from a swine lineage known as 

“Eurasian avian.” A change to a hemagglutinin against which the 

human population has no immunity (“antigenic shift”) causes these 

periodic global pandemics. The 2009 H1N1 influenza pandemic 

revealed the importance of swine and humans to the global spread 

of influenza viruses. As a result of the recent pandemic, several 

important areas of research and surveillance were undertaken in 

the hopes of answering questions about influenza virus evolution, 

adaptation , and transmission. 

So, how does this process of reassortment occur? In general, there 

is a functional “species” barrier to virus transmission between 

people and animals. This barrier exists, in part, because avian 

influenza viruses preferentially bind receptors expressed on bird 

enteric cells and human viruses preferentially bind receptors 

expressed on cells in the human respiratory tract. Pigs express 

some avian- and human-type receptors and that may allow 

infection with avian, human and swine influenza viruses. As such, 

pigs can serve as hosts in which avian viruses adapt to replicate in 

mammals. For example, in the late 1970s, an avian H1N1 virus of 

waterfowl-origin entered the pig population of Europe and soon 

became the dominant cause of influenza among European pigs. 

Subsequently, these avian H1N1 viruses were also sporadically 

detected in people in Europe and Asia, and ultimately contributed 

gene segments to the 2009 H1N1 human pandemic virus some 40 

years later. Pigs are hypothesized to serve as the “mixing vessels” 

in which reassortment between avian and human influenza viruses 

can take place. To a lesser extent this occurs in other mammalian 

species such as dogs, even though no dog to human transmission 

events have been documented. The focus of such reassortment 

has historically been in Southeast Asia, the proposed “influenza 

epicenter,” because cultural practices in this region bring pigs, 

people and waterfowl and other avian species into close contact 

with one another. However, it is now clear that influenza virus 

reassortment in pigs can occur anywhere in the world, as evidenced 

by the 2009 pandemic precursor viruses isolated from pigs in 

Mexico. Genetic mixing between IAV occurs frequently among 

swine, and other species. For example, interspecies transmission 

of influenza viruses from pigs to domestic turkeys has been 

recognized on numerous occasions. In contrast, transmission of 

influenza viruses between pigs and domestic chickens and other 

fowl, and vice versa, is very rarely reported.

Reducing interspecies transmission of 

influenza viruses

It is in the best interest of both human public health and swine 

health to minimize interspecies transmission of influenza viruses 

(i.e. from pigs to people, from people to pigs, from birds to pigs 

and from pigs to birds).

Interspecies transmission among 

pigs and people

Zoonotic transmission of IAV from pigs to swine workers may 

go unrecognized or be misdiagnosed as typical human influenza 

viruses because the seasonal patterns of human and swine influenza 

disease largely overlap (late fall through winter) and confirmatory 

diagnosis requires subtype-specific and/or genomic testing to 

distinguish seasonal from zoonotic influenza virus infection. 

Uniquely, however, many zoonotic influenza cases have been 

associated with exposure at agricultural fairs during the summer 

months. These cases typically occur in children exhibiting swine 

or among people of higher risk for influenza, making these cases 

easier to recognize and trace back to swine exposure. In addition, 

the impact of transmission of influenza viruses from people to 

pigs should not be underestimated. Human seasonal IAV have 

sporadically made incursions into pig populations around the 

world. Human seasonal H1N1 and H1N2 viruses were introduced 

into U.S. swine in the early 2000s and human seasonal H3N2 

viruses have spilled over into pigs repeatedly since 1968, with 

recent incursions occurring during the 2000s in China and Europe 

and during the 2010s in the United States. And since 1998, H3N2, 

H1N2 and H1N1 viruses circulating and causing disease in swine 

in the United States have contained human influenza virus genes. 

The human pandemic H1N1 virus from 2009 continues to be 

repeatedly transmitted from people back to pigs. 
The following steps are potentially useful measures to reduce transmission of 

influenza viruses between pigs and people in the swine industry:

Worker biosecurity: Provide clothing and boots for 

workers that are worn only within the pig housing units, thus 

eliminating the chance to carry IAV into housing units from 

external sources. No contact with swine from other sources or 

with poultry is recommended for at least a 24 hour period before 

entry, along with a full body shower in between.

Basic hygiene practices: Workers should change clothes 

prior to leaving swine barns for office facilities, food breaks 

or their homes. In addition, hand-to-face contact should be 

minimized and hand-washing stations should be available and 

used prior to leaving each room or area and before returning to 

office or break rooms. Influenza viruses are spread not just by 

inhalation of aerosolized virus, but also by eye and nose contact 

with droplets of respiratory secretions. During outbreaks of 

ILI in swine, proper use of respirators can mitigate risk of 

transmission from pigs to workers.

Sick-leave policies: To further reduce the chances for 

infection of pigs with human lineage influenza viruses, the 

farm owner should provide sick-leave policies for employees 

that encourage them to remain away from work when they are 

suffering from acute respiratory infections. People typically 

shed influenza viruses for approximately 3-7 days, with the 

period of peak shedding correlated with the time of most severe 

clinical illness. Influenza-like illness (ILI) is typically defined as 

fever, cough, congestion, sore throat, and muscle aches. Workers 

that exhibit ILI after working with ill swine should seek medical 

care and disclose the swine contact to the medical professionals.

Pursue alternative employment: People with medical 

conditions that predispose them to severe influenza illness are 

recommended to avoid contact with swine, whether at public 

venues or through occupational exposure.

Influenza virus vaccination of swine farm workers: The 

vaccines produced on a yearly basis for the human population 

contain only human, not swine, strains of influenza viruses. 

Nonetheless, vaccination of farm workers may reduce the 

amount of virus shed by infected people during human 

influenza outbreaks, and thereby limit the potential for human 

influenza virus infection of their pigs.

Ventilation & environmental conditions: Ventilation 

systems in containment production facilities should be designed 

to minimize re-circulation of air within animal housing rooms. 

This is important to reduce the exposure of pigs to viruses 

from other pigs, to reduce their exposure to human influenza 

viruses, and conversely, to reduce exposure of workers to swine 

influenza viruses. Stressful environmental conditions or too few 

air exchantges contribute to reoccurrence of disease outbreaks 

caused by endemic strains.

Influenza virus vaccination of pigs: While the IAV vaccines 

used in swine today may not induce sterilizing immunity, nor 

completely eliminate clinical signs of infection, vaccination of 

pigs can reduce the levels of virus shed by infected animals, 

and thus reduce the potential for human exposure and zoonotic 

infections.

Interspecies transmission among pigs and birds: The 

global reservoir of IAVs in waterfowl, the examples of infection 

of pigs with waterfowl-origin influenza viruses, the risks 

for reassortment of avian viruses with swine and/or human 

influenza viruses in pigs, and the risk for transmission of 

influenza viruses from pigs to domestic poultry (especially 

turkeys) all indicate that contact between pigs and both wild and 

domestic fowl should be minimized.

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The following factors are potentially useful to reduce transmission of influenza viruses 

between birds and pigs:

• Bird-proofing: All doorways, windows and air-flow vents in swine housing units should be adequately sealed or screened to 

prevent entrance of birds. 

• Water treatment: Do not use untreated surface water (because of waterfowl fecal contamination with influenza viruses) as either 

drinking water or water for cleaning in swine facilities. Likewise, it may be prudent to attempt to minimize waterfowl use of farm 

lagoons/ponds.

• Separation of pig and bird production: Do not raise pigs and domestic fowl on the same premises.

• Feed security: Keep pig feed in closed containers to prevent contamination with feces from birds and other peridomestic wildlife. 

Vigilance regarding the quality and origin of feed ingredients is recommended for feed security in general.

Note: These recommendations clearly cannot apply to production units in which pigs are raised outdoors. Outdoor housing places pigs at 

increased risk for infection with influenza viruses from multiple sources.
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Suggested reading and references

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Transmission%20at%20Swine%20Exhibitions_2018.pdf

Abente, E.J., Gauger, P.C., Walia, R.R., Rajao, D.S., Zhang, J., 

Harmon, K.M., Killian, M.L., Vincent, A.L., 2017. Detection and 

characterization of an H4N6 avian-lineage influenza A virus in 

pigs in the Midwestern United States. Virology 511, 56-65.

Bliss N, Stull JW, Moeller SJ, Rajala-Schultz PJ, Bowman AS. 

Movement patterns of exhibition swine and associations of 

influenza A virus infection with swine management practices. J 

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javma.251.6.706.

Bowman AS, Nelson SW, Page SL, Nolting JM, Killian ML, 

Sreevatsan S, Slemons RD. Swine-to-human transmission 

of influenza A(H3N2) virus at agricultural fairs, Ohio, USA, 

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Chamba Pardo, F.O., Alba-Casals, A., Nerem, J., Morrison, R.B., Puig, 

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Corzo, C.A., Allerson, M., Gramer, M., Morrison, R.B., 

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